Guide to Injecting Services with Dependencies in Angular 2 (Using Ionic 2/Angular 2/Typescript)

As part of my project, I am developing a sample application that connects to a websocket server in Ionic 2 using Typescript. You can find the repository here.

The main requirement is to establish the websocket connection when the application starts up.

To create the connection, I am utilizing the angular2-websocket library.

Here are some helpful references:

However, I encountered an error message stating: "Cannot resolve all parameters for '$WebSocket'(String, Array, ?). Make sure that all the parameters are decorated with Inject or have valid type annotations and that '$WebSocket' is decorated with Injectable."

Below is a snippet of the code:

app.ts

import {App, Platform} from 'ionic-framework/ionic';
import {TabsPage} from './pages/tabs/tabs';
import {ConnectionService} from './framework/connection/connection-service'
import {$WebSocket} from 'angular2-websocket/angular2-websocket';
import {bootstrap} from 'angular2/platform/browser';

// Other imports...

@App({
  template: '<ion-nav [root]="rootPage"></ion-nav>',
  config: {}
})
export class MyApp {
  
    // Constructor...
    
}
bootstrap(MyApp, [$WebSocket, ConnectionService]);

connection-service.ts

import {Injectable, Component, Inject} from 'angular2/core';
import {$WebSocket} from 'angular2-websocket/angular2-websocket';
import {bootstrap} from 'angular2/platform/browser';

@Injectable()
export class ConnectionService {

    private _status: number;

    // Constructor and other methods...
    
}
bootstrap(ConnectionService, [$WebSocket]);

Answer №1

I was able to solve my issue by utilizing the @App() Annotation's provider field instead of relying on bootstrap

bootstrap(ConnectionService, 
    [provide($WebSocket, useValue: new WebSocket("ws://echo.websocket.org")]);

For example:

@App({
  template: '<ion-nav [root]="rootPage"></ion-nav>',
  config: {},
  providers : [ provide( $WebSocket, { useValue: new $WebSocket("ws://echo.websocket.org") }), ConnectionService]
})

A working demonstration can be accessed here

Answer №2

It is clear that the $WebSocket function requires specific parameters such as a string, an array, and another value, making it non-injectable directly.

To work around this limitation, you can utilize:

import {Injectable, Component, Inject, provide} from 'angular2/core';

bootstrap(ConnectionService, 
    [provide( $WebSocket, { useValue: new $WebSocket("ws://echo.websocket.org") })]);

If the string literal does not suit your needs, there are other options available, such as using a factory method instead.

Answer №3

It appears that in order to properly utilize dependency injection in your Angular application, you should consider injecting the $WebSocket service rather than instantiating it manually. You can achieve this by removing it from the bootstrap and creating an instance of it within the constructor of your service:

@Injectable()
export class ConnectionService {
  private _status: number;
  private connection: $WebSocket;

  constructor() {
    this.connection = new $WebSocket("ws://echo.websocket.org");
  }
}

When working with Angular's dependency injection system, it's important to allow Angular to handle the instantiation of services that need to be injected into constructors.

In your specific case, it seems like Angular is having trouble resolving the parameters for the $WebSocket service during instantiation. It's possible that the constructor for $WebSocket requires three parameters, causing a resolution issue at the initialization stage. Can you provide more information on the required parameters for the $WebService's constructor?

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